The 78-year-old was filming The Imaginarium of Doctor Parnassus with Ledger in the British capital last week.
He says, "We're all a bit befuddled at the moment. It's so sad.
"Heath did have a terrible, lingering bug in London, and he couldn't sleep at all. We all--I thought he'd probably got walking pneumonia, which they seem to think he had. Of course, I don't really know, but that's the latest...
"The last few days we were shooting outside a pub. Always outside. Cold as bejesus. You know how damp it gets in London. And at night the temperature drops horribly, and that little breeze gets up. You have to wear tons of stuff...
"Shooting at night in a big city, it's not easy... We were working in such dire conditions in London, outside every night in the cold. Which may have contributed somewhat to his health. We were all armed with antibiotics all the time. It wasn't exactly joyous, except that the film itself was fun to do.
"Heath's death is sort of literally unbelievable, because apart from the sleeping, he was in such good form... There was a sweetness about him. He was a very charming and gentle guy, actually... I just left a very laughing, happy fellow, practically a few minutes ago."
Ledger was found dead in his New York apartment on Tuesday.
An autopsy carried out on Wednesday failed to reveal how the actor died, although police believe his death could be drug-related, as five different bottles of prescription drugs were found around his apartment.
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